The invention relates generally to combination weighing systems and deals more particularly with a combination weighing system which weighs out quantities of two or more different products for inclusion in a single package.
In packages containing different types of products, usually the precise amount of each product in the package is not critical; however, it is desirable that each amount approximate a predetermined, fractional weight and it is generally necessary that the total weight of all products be not less than a predetermined target weight. Also, it is frequently desirable that the different components exist in mixed form within the package.
It is known in the industry to weigh out two or more separate quantities of different products, each weighing process being carried out with a single scale, and then deliver the quantities of products to a mechanical mixer having mixing blades. After mixing, the batch is dispensed into a package. A problem with this system is that the blades may damage the product during mixing. Another problem stems from the fact that each scale operates independently of the other and therefore, is required to weigh out a quantity of product at or above a fractional target weight with a resultant giveaway, the difference between the actual weight of the quantity and the fractional target weight. Consequently, on the average, the total weight exceeds the package target weight by an amount equal to twice the average giveaway associated with scales of the kind. Still another problem with this system is that the mixer adds to the size, and particularly the height of the weighing and mixing system.
Combination weighing machines which weigh out a quantity of a single type of product are known in the industry and in general, operate at high speed and provide little giveaway. Such machines employ a multiplicity of weighing scales, a computer which searches the scales to select a combination of them having a combined weight near a target weight, usually at or above the target weight, and a common discharge chute which receives product dumped from the selected scales and guides it to a packaging device. There are many known processes for searching a number of scales to locate a combination which satisfies weight range criteria. In one process, a combination weighing system includes several scales and a binary counter which generates 2.sup.N -1 counts from "N" output ports. Each output port of the counter represents one of the scales so that each count indicates a different combination of scales. Another combination searching process is disclosed in U.S. Pat. No. 4,466,500 to Mosher, et al. in which a combination generator searches sequences of combinations of scales, with certain steps of these sequences being omitted from the sequence if they involve subcombinations previously searched and found to be at or above the target weight. The Mosher '500 patent is assigned to the assignee of the present invention and is adopted by reference as part of the present disclosure.
Occasionally, combination weighing systems "lock-up", a condition in which no combination of scales satisfies the package weight criteria. To overcome lock-up, product may be added to or removed from one or more scales and another search performed. Often the addition or removal of product is performed manually.
A general aim of the invention is to provide a weighing system which weighs out quantities of two or more different types of products in correct proportions and amounts totaling the target weight for a single package.
A more specific aim of the invention is to provide a weighing system of the foregoing type which minimizes the giveaway.
Another specific aim of the invention is to provide a weighing system of the foregoing type which also mixes together the two or more different types of products prior to packaging.
Another specific aim of the invention is to provide a system of the foregoing type in which the mixing operation does not harm the product.
Still another specific aim of the invention is to provide a system of the foregoing type in which the means for mixing is inexpensive to implement.
Still another specific aim of the invention is to provide a combination weighing system of the foregoing type in which the incidence of lock-up is small.